Naikun Sun

Effect of microstrain on the magnetism and magnetocaloric properties of MnAs0.97P0.03

In the compound MnAs0.97P0.03, prepared by mechanical milling, a large microstrain of 0.68%, calculated by quantitative x-ray diffraction analysis, induces a recoverable helimagnetic state at low temperatures and suppresses the temperature/field-induced orthorhombic-hexagonal phase transition. This leads to a remarkable reduction of both the thermal and the magnetic hysteresis at the Curie temperature, TC. Around the helimagnetic-ferromagnetic transition temperature and at TC, a large inverse magnetocaloric effect (MCE) with magnetic entropy change ΔSm of 5.6 J/kg K at 208 K and a normal MCE with ΔSm of −4.4 J/kg K at 253 K for a 5 T field change are observed. After annealing, MnAs0.97P0.03 exhibits a large MCE near room temperature with ΔSm of ∼14 J/kg K for a field change from 0 to 5 T.

High magnetic-refrigeration performance of plate-shaped La0.5Pr0.5Fe11.4Si1.6 hydrides sintered in high-pressure H2 atmosphere

La(Fe, Si)13 hydride is regarded as one of the most promising room-temperature refrigerants. However, to use the alloys in an active magnetic regenerator machine, it is vital to prepare thin refrigerants. In this work, a high H2 gas pressure of 50 MPa was employed to suppress the desorption of hydrogen atoms during the sintering process of plate-shaped La0.5Pr0.5Fe11.4Si1.6 hydrides. At 330 K, a high-density sintered thin plate shows a large magnetic-entropy change ΔSm of 15.5 J/kg K (106 mJ/cm3 K) for a field change of 2 T. The volumetric ΔSm is almost twice as large as that of bonded La(Fe,Si)13 hydrides. Favorably, hysteresis is almost absent due to the existence of micropores with a porosity of 0.69% which has been analyzed with high-resolution X-ray microtomography.

Effect of Annealing on the Microwave-absorption Properties of Ni/Al2O3 Nanocomposites

Ni/Al2O3 nanocomposites were prepared by the mechanochemical synthesis method. The annealing process enlarges the grain size of both the metal Ni and insulating Al2O3 in the as-milled nanocomposite and leads to an increase of the saturation magnetization and a decrease of the surface anisotropy. An optimal reflection loss (RL) of −23 dB is obtained in the as-milled nanocomposite at 17.8 GHz for an absorber thickness of 6.6 mm. The annealed sample exhibits a RL exceeding −20 dB in the whole Ku-band for an absorber thickness of 6.6–9.7 mm with an optimal RL of −54.7 dB at 13.2 GHz for a layer thickness of 9.3 mm. The excellent microwave-absorption properties are a consequence of a proper match of the dielectric and magnetic losses.

Effect of B-doping on the structure and magnetocaloric properties of plate-shaped La0.6Pr0.4Fe11.4Si1.6Hx sintered in high-pressure H2 atmosphere

Plate-shaped La0.6Pr0.4Fe11.4Si1.6B0.2Hx bulk samples have been achieved with sintering in a high-purity H2 atmosphere at 50 MPa. The effect of B-doping on the structure, magnetism and magnetocaloric properties of the plate-shaped hydrides has been systematically explored. The results show that B-doping unfavorably leads to a remarkable increase of Fe2B during the sintering process and has not helped much in the 1:13 phase stabilization and/or in the magnetocaloric properties. At 340 K, a high-density sintered thin plate shows a large magnetic-entropy change ΔSm of 16.2 J/kg·K and a favorable small hysteresis of 0.6 J/kg for a field change from 0 to 5 T. High-resolution X-ray microtomography analysis shows that micropores exist in the thin plates causing a porosity of 0.26% and leading to a remarkable reduction of the hysteresis. This work opens an effective route for synthesizing thin magnetic refrigerants of La(Fe, Si)13Hx hydrides.

Effect of B-doping on Microstructure and Magnetic Properties of SmFe10Mo2 Alloy

Ingots of Sm Fe10Mo2 and Sm Fe10Mo1.5B0.5alloys were made by arc-melting method and followed by annealing treatment, with which nanocrystalline powders of the alloys were prepared by milling.While the effect of B- doping and ball milling on the phase composition and magnetic properties of the bulk and the nanocrystalline particles of Sm Fe10Mo1.5B0.5alloy was investigated. It has been found that Bdoping did not alter the crystallographic structure of the phase Th Mn12 in the alloy, but remarkably increased its Curie temperature from 270 oC to 334 oC. Due to the inhomogeneity of alloy compositions, correspondingly, in its thermal magnetic curve there exist two abrapt changes related with phase transformation. The post ball milling process could induce the occurrence of precipitates of Mo and α-Fe, which in turn reduce the amount of the Th Mn12 phase and promote the formation an non magnetic phase of Mo2 Fe B2. As a result, with the progress of ball milling process the intrinsic coercive force decreased and the saturation magnetization increased first and then decreased of the Sm Fe10Mo1.5B0.5powders. However, the nanocrystalline particles of Sm Fe10Mo1.5B0.5alloy milled for 0.5h exhibit optimal magnetic properties:iHc=2.2k Oe and Ms=4.3k Gs.

MAGNETOCALORIC EFFECT OF La0.9Ce0.1Fe11.44Si1.56Hy ALLOY AND POWER BONDED BLOCK

Recently, La(Fe, Si)13based magnetic refrigeration materials have been widely explored due to the advantages of giant magnetocaloric effect (MCE), tunable Curie temperature (TC), low cost of raw materials and excluding deleterious elements compared to other room-temperature giant MCE materials such as Gd5(Ge1-xSix)4, MnFeP0.45As0.55 and MnAs based compounds. In this work, in order to shift the TC to around room temperature and maintain the large MCE, the method of absorbing hydrogen was employed. La0.9Ce0.1Fe11.44Si1.56 hydride was prepared by saturated hydrogen absorption and then hydrogen contents and TC of the hydrides were controlled by subsequent dehydrogenation at different temperatures (Td=200~250 °C for 3 h). The phase structure and magnetocaloric effect were investigated. The results show that the samples possess the cubic NaZn13-type structure with a small *国家自然科学基金项目51261001和辽宁省自然科学基金项目2013020105资助 收到初稿日期: 2014-08-28,收到修改稿日期: 2015-01-15 作者简介:慕利娟,女, 1978年生,副教授,博士生 DOI: 10.11900/0412.1961.2014.00473 第762-768页 pp.762-768

Influence of annealing treatment on Microstructure and Waves Absorption of Ni/TiO2 Nanocomposites

Ni/TiO2nanocomposite was prepared by mechano- chemical synthesis. The as- milled nanocomposite consists of hexagonal-structure rutile TiO2and face-centered cubic structure Ni. Annealing treatment decreases both the lattice defects and the internal stress, while remarkably increases the grain size of metal Ni and the saturation magnetization. After annealing treatment the intrinsic emission, free exciton emission and bound exciton emission peaks of TiO2exhibit a red shift, for which the reason may be the distortion of the band structure. For the as-milled nanocomposite an intense nonlinear dielectric resonance occurs at 15 GHz and a clear Cole-Cole semicircle was observed. Meanwhile, a reflection loss(RL) exceeding-10 dB in a frequency range of 14-16 GHz and a maximum value of RL as large as-32 dB are obtained for an 8 mm thick absorber. After annealing treatment the intensity of the natural resonances at 2.8 and 5.2 GHz has been significantly strengthened, leading to a remarkable improvement of microwave-absorption properties in the frequency range 2-6 GHz.

Effect of Annealing on Microwave-absorption Properties of Fe/Al2O3 Nanocomposite Prepared by Ball-milling

An Fe/Al2O3nanocomposite was prepared by high energy ball milling method, and which then annealed at 700oC for 0.5 h in Ar atmosphere. The effect of annealing on the microstructure, magnetism and microwave- absorption properties of Fe/Al2O3nanocomposite has been investigated. It follows that the annealing resulted in increasing grain size, decreasing lattice defects and releasing the residual microstrain; meanwhile, decreasing the intensity of photoluminescence peaks of Al2O3significantly, therewith the peaks at 398 nm and 484 nm exhibit a blue shift. An optimal reflection loss of-11.4 dB at 15.5GHz was measured for a 7.7 mm thick layer of as-milled composite powder mixture with paraffin. However, the annealing treatment could remarkably enhance the dielectric loss, magnetic loss and microwave absorption performance of the composite, leading to an optimal reflection loss of-35.5 dB at 17 GHz for a layer thickness of 6.4 mm for the annealed composite.

Structure and Magnetostriction of Tb0.7Pr0.3FeX Prepared by Solid-State Synthesis

Mechanical alloying (MA) was used to prepare pseudobinary high Pr-content Tb0.7Pr0.3Fex alloys. The structural and magnetoelastic properties were in investigated by means of x-ray diffraction and a standard strain technique. A single (Tb,Pr)Fe2 Laves phase with MgCu2-type structure for the composition Tb0.7Pr0.3Fe1.80 was synthesized by MA and subsequent annealing process. The high magnetostriction of 600 ppm was achieved at 10 kOe for the epoxy-bonded Tb0.7Pr0.3Fe1.8 composite.